Optical communication systems generally operate by switching a plurality of signals over a multitude of routes for communication and other processing functions. To accomplish this end, directional couplers have been developed to perform the switching function under optical or electrical control.
One such directional coupler switch in prevalent use today is fabricated in LiNbO.sub.3 material using an alternating .DELTA..beta. or reversed .DELTA..beta., split electrode structure. See commonly assigned U.S. Pat. No. 4,012,113. This type of directional coupler switch includes an input transition region wherein two waveguides are bent toward each other to be in close proximity, an interaction region wherein optical energy transfer is electrooptically effected, and an output transition region wherein the two waveguides diverge from each other to eliminate interwaveguide coupling. In the interaction region, electrodes are split and separated by an insignificantly small gap. For this type of device, it is known that voltage adjustments can be employed to reduce crosstalk within certain limits in the straight-through ("bar") and crossover ("cross") states of the switch. However, crosstalk reduction techniques such as voltage adjustment on the reversed .DELTA..beta. electrodes or the use of uniform .DELTA..beta. electrode structures have only addressed crosstalk introduced in the interaction region of the directional coupler switch while essentially neglecting crosstalk introduced in the transition regions. As a result, such directional coupler switches have experienced intolerable crosstalk levels when the switches are cascaded in arrays.